Icheb and Leucon next to a Brunali genetic resequencer
A genetic resequencer was a device used to alter DNA.
The Brunali civilization in the Delta Quadrant built a genetic resequencer from parts of damaged starships, which they used to create genetically-engineered crops capable of surviving on the harsh conditions of their devastated homeworld. (VOY: "Child's Play")
Though not explicitly stated, it seems likely that this device was also used to engineer the children who produce the anti-Borg spaceborne virus, such as Icheb.